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Gallium Solder Alloys: Questions & Answers

I’ve received numerous questions about using gallium liquid metal alloys, so thought I’d present some of my answers for all.  The customer questions are in black, and my responses in red.     

  1. Indium’s product data sheet, "Indalloy Metals Liquid at Room Temperature" mentions that "any liquid metal will wet another clean metal surface". Can you please elaborate on the conditions that make such wetting possible? 

As far as wetting, gallium alloys (Indalloy 46L, Indalloy 51E, Indalloy 60, etc.), coat nearly any organic, ceramic, or metal surface. It is difficult to come up with materials used in packaging and processing these alloys that come clean of the alloy after use.   The physics of this affinity is unknown, however the low melting point and surface tension are the source/consequence.

 

Many metals will form some alloy with the gallium, but the solubility in gallium is limited. The gallium wets the surface and forms a solid gallium-alloy layer, which then acts as a diffusion barrier. In the case of aluminum, gallium forms an amalgam which ends up consuming a large volume of aluminum before a stable solid layer.

  1. Do you assume an oxygen atmosphere (gallium oxide does wet most surfaces)? Or, to the opposite, do you refer to the wettability of plasma-clean metal surfaces?

 

This wetting behavior relates to surfaces in air or vacuum. Surfaces do not have to be atomically clean. Gallium oxide does form as a film over the surface of a pool of these alloys. However, it does not diminish the wetting behavior. 
  1. Can you let me know if Indium Corporation sells droplet dispensing equipment for liquid solders/metals?

 

Liquid metals are typically offered in a syringe and various needle gauges can be screwed to the tip to adjust the dispensed droplet size. For dispensing thin layers of liquid metal, such as for a thermal interface, we recommend PVA selective coating equipment.

 

  1.  We are interfacing our thermoelectrics with both steel and aluminum and the hottest point will be around 600°C. Can liquid metals be used for this?

InGa alloys are selected typically because they remain liquid at all times, negating any contact resistance between substrates.  The typical application for these has temperatures up to approximately 100°C or slightly higher.  At these temperatures we have noted some aluminum corrosion by the gallium, which will only become worsened by your elevated temperature. 

There are other alloys which do not contain gallium and have only a slightly higher temperature, such as the Bi/In alloys. At 70°C, the eutectic composition of this alloy will also be liquid in phase, however is much less reactive with your bonding metals, and therefore may be better suited.